In general, when a panoramic image of the tooth row is taken with a dental X-ray photographic device, cervical vertebrae appear in the place of the anterior teeth of the panoramic image, making that part white.
Therefore, conventionally, a technique is known to intensify an X-ray output only when photographing the anterior teeth, for ensuring enough X-ray incidents on the image-receiving surface of the X-ray detector. This technique can be classified into two methods. The first method is to intensify the X-ray output uniformly in the section corresponding to the anterior teeth by a predetermined value. The second method is to continuously monitor X-ray incidents on the image-receiving surface of the X-ray detector during photographing and controlling the X-ray output so that X-ray incidents are kept with a predetermined value at all times. It is impossible with the first method to react to different shapes of cervical vertebrae and X-ray permeability by each subject, but with the second method, it is possible to react thereto. However, it is necessary to increase the X-ray dose received by the subject with either one of the methods and that is not preferable.
On the other hand, for example, X-ray photographic devices disclosed in Patent Document 1 and Patent Document 2 construct a panoramic image in which the influence of the cervical vertebrae is eliminated by performing arithmetic processing on the image acquired by X-ray photographing. Hereinafter, an example of eliminating the influence of the cervical vertebrae by a conventional X-ray photographic device will be described, with reference to FIG. 11. FIG. 11 shows a mandibular tooth row 901 and a cervical vertebra 902, as an example. In addition, an imaging plane to construct a panoramic image of the tooth row is indicated by reference numeral 903. Here, each of reference numerals 905 and 906 shows an example of an incident X-ray beam from a predetermined direction to the tooth row 901. Further, an imaging plane to construct a panoramic image of the cervical vertebrae is indicated by reference numeral 904, in order to eliminate the influence of the cervical vertebrae from the panoramic image of a tooth row. The imaging plane 904 sections through the cervical vertebra 902 and the right and left mandibular angles 908. Further, reference numeral 907 indicates a trajectory of the pivot arms arranged with a X-ray generator and a X-ray detector, respectively, at the X-ray photographic device, facing each other and having the tooth row 901 in-between.
The conventional X-ray photographic device first detects the X-rays transmitted through the tooth row 901 by frames and overlays a sequence of image frames to form a panoramic image I1 of the tooth row on the imaging plane 903. Next, the conventional X-ray photographic device forms a panoramic image I2 of the cervical vertebrae on the imaging plane 904. Then, the conventional X-ray photographic device simulates on a computer how the panoramic image I2 of the cervical vertebrae is blurred when it is projected onto the imaging plane 903, to acquire a panoramic image I3 projecting the cervical vertebrae. The conventional X-ray photographic device subtracts the panoramic image I3 projecting the cervical vertebrae from the panoramic image I1 of the tooth row obtained above, to acquire a panoramic image I4 of the tooth row without the influence of the cervical vertebrae.